vsftp behind firewall

[Kiss Gábor <kissg dataminer hu>]

Dear All,

we have a Linux server behind an adsl router. The router can forwarding the
linux ftp port. The ftp server in our intranet is working fine. From outside
(from internet) I can log in to the ftp, but if I made an ls command, it
hangs up. After timeout I got the following error:
500 Illegal port command.  I have tried it passive and not passive mode too.

Can somebody help me?

Thanx
Gábor